Torsion springs are typically made of either wound wire or coiled ribbon. With few exceptions, they possess a constant and nearly linear spring rate.
Torsion bars used in vehicle suspensions use a long prismatic tube as a spring. Their stiffnesses are not changeable nor able to generate the variety of stiffness envelopes (linear, hardening, softening, non-linear, and negative) that we claim.
What is needed is a torsion spring with adjustable spring rates. Moreover, what is needed is customizable non-linear of spring rate.